Burner assemblies which combust gas, such as propane and natural gas, are well known and widely applied. For example, boilers, furnaces, kilns, incinerators, dryers, and food processing equipment all commonly rely upon the heat generated by such combustion for proper operation.
Prior art burner assembly designs have been created to mix a combustible gas with air and provide a spark for the purpose of starting. Extensive attention has been directed to finding proper mixing ratios and to creating apparatus for obtaining such ratios to most efficiently burn the gas while maximizing BTU output.
The airflow characteristics influence BTU output, flame stability, CO and NOx emissions. BTU output is a measure of the strength of the flame and its resulting heat output, and is a function of, among other things, the amounts of air and gas combined and the ratio at which they are combined. Flame stability relates to the maintainability and controllability of the flame. If the gas/air ratio becomes too rich or too lean, the flame can be lost or can burn inefficiently. CO and NOx emission control is critical in complying with various environmental regulations. If the flame is not suitably confined, shaped, and directed, all three of the foregoing characteristics will be adversely affected. 
Prior art burner assembly designs use a damper for controlling the amount of combustion air entering the burner. Such dampers are typically of the louver type or butterfly valve type and can be opened and closed in various degrees to control the amount of air entering the burner. Because the ratio of combustion gas to combustion air highly influences burner efficiency and burner emissions, precise control of the amount of air entering the burner is critical.
However, both the louver type and the butterfly valve dampers cannot precisely control the amount of air entering the burner. When one or more louvers of a louver type damper are closed, air can seep into the burner from small gaps between each pair of louvers and from the sides of each louver. Even when the louvers are closed, air can seep into the burner from the sides of each louver. Accordingly, precise control of air entering the burner cannot be achieved with louver type dampers. Similarly, a rotating gate of the butterfly valve cannot precisely control the amount of air entering the burner because of air leaks between the rotating gate and air ports, which the gate opens and closes.